The kinetics of alkaline deacetylation of cottonwood (Populus deltoides) was experimentally studied. In order to minimize resistance to mass and heat transfer, wood meal (40/60 mesh) was employed. Wood samples were isothermally treated with alkali which was maintained at constant concentration. The temperature range was 50-90°C and the alkali concentration was up to 9.6 g 1 . A simple expression relating acetyl content in wood, temperature and alkali concentration in the liquid phase was found. The rate equation was of second order with respect to the acetyl content and of order 1.35 for the alkali concentration. The rate constant of the equation followed the Arrhenius law. A total deacetylation rapidly took place at high temperature, even at low alkali concentration. At low temperature, the deacetylation was completed in a few minutes when the alkali concentration was the highest in the range studied.
The impregnation pattern of alkali in fresh Cottonwood is analysed. Isothermal process, medium alkaliconcentration, temperature below 100 o C and tangential direction are considered. The degree of deacetylation in the wood is taken as an indicator of the whole chemical action of the alkali. Profiles of alkali concentration, alkali content, liquid content and acetyl content in the wood are experimentally determined. The results show that wood behaves rather like a glassy polymeric solid placed in contact with a solvent. An advancing boundary zone is established which separates an intact inner part from the outer swollen zone. Alkali concentration and acetyl groups content profiles can approximately be described by a model used to analyse the reaction of solids: the shrinking core model. The chemical reaction in the front is the controlling mechanism. The swelling of the cell walls together with the occupation of the hollow cavities of the wood by the liquor make it possible for the wood liquid content to increase up to almost 3 g liquid g wood -1 .The procedure here adopted and the impregnation pattern proposed are valuable tools to analyse the effect of wood direction and process variables such as temperature, alkali concentration and initial wood moisture. They could also be very useful to build a model for alkaline impregnation of wood under moderate conditions.
The low temperature alkaline treatment of Cottonwood (Populus deltoides) was studied by applying a factorial experimental design. The effects of treatment time and alkali concentration on alkali consumption, as well as treatment yield, carboxyl groups and acetyl groups contents were investigated. Relationships between these contents and wood swelling measured by the water retention value (WRV) were also analyzed. In the whole alkali-concentration range under study, the deacetylation process was responsible for a great part of the alkali consumption. Fitted curves show that the swelling obtained is always increasing despite the fact that the carboxyl groups content attains its higher levels at intermediate times. The unique relationship found for the WRV of the wood and its acetyl groups content, indicates that the deacetylation, rather than the acid group content, is the process determining the wood swelling brought about by the alkaline treatment.
Summary
In order to analyse the fundamentals of alkaline chemi-mechanical pulping of hardwoods, the
chemical state of the wood was related to both the swelling level of fibres and the papermaking
properties of pulp. Wafers of poplar wood were alkali treated following a factorial experimental
design for two variables: temperature and alkali concentration. Treated wafers were hot defibrated
in a 300-mm disk mill at 15% consistency, and then refined in PFI mill at 20% consistency. Results
show how fibre swelling gradually increases as alkaline action is increased. The significant improvement
in tensile and tear strength of the pulp can, in great part, be ascribed to the development
of fibre bonding capacity. A limited effect of ion content on cell wall swelling was found.
Swelling correlates well with deacetylation level, and is a major factor in determining the tensile
strength and scattering ability of the pulp.
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